With some decentralized systems, an air-side economizer is an option, if not an energy code requirement (check state code for criteria). The air-side economizer uses cool outside air to either assist mechanical cooling or, if the outside air is cool enough, provide total cooling.

It requires a mixing box designed to allow 100% of the supply air to be drawn from outside. It can be a field-installed accessory that includes an outside air damper, relief damper, return air damper, filter, actuator, and linkage.

Controls are usually a factory-installed option. Self-contained units usually do not include return air fans. A barometric relief, fan-powered relief fan, or return/exhaust fan may be provided as an air-side economizer.

The relief fan is off and discharge/ exhaust dampers are closed when the air-side economizer is inactive.